EP0693357B1 - Method for extrusion blow moulding hollow bodies of thermoplastic material - Google Patents

Abstract

The thermoplastic extruder's (2) orifice (4) is controlled by the wall thickness program (3). The preform (1) is blown to a hollow body (7) in the mould (6), removed, and any adherent fragments are taken out. The time t1st to extrude a given length, is measured; difference at DELTA G from the desired value Gsoll; this is stored, paired with DELTA t values. The pairs control ns and sa in a following cycle. It measures the time to grow the preform, even before it is blown to shape. This is largely controlled by the extrusion gap and extruder screw speed, for given material properties. The weight of the vessel, the material distribution in the hollow blown body, and the wall thickness are controlled. The process in this case produces a particularly marked stretching of the preform, a rectangular vessel being depicted. The parameters are particularly quickly stabilised. Any weight variation is fed back to the next cycle. All the key process parameters are inter-related to the programmed wall thickness. Because the parameters are inter-related, methods relying on individual adjustment and measuring the effects in isolation are obviated. Further process details are provided, and examples of the necessary corrective action, in terms of screw speed alteration, required by measured extrusion times, are tabulated for a practical case. <IMAGE>

Description

The invention relates to a method for extrusion blow molding of hollow bodies made of thermoplastic, in the case of the tubular preforms by means of a Screw extruder from the according to a wall thickness program controlled nozzle gap of an extrusion head extruded and in the following work cycles the extruded Preforms expanded into hollow bodies in a blow mold, the hollow body formed and adhering waste slugs be removed.

The wall thickness program controlling the nozzle gap has at least a pronounced maximum of function, the one Preform section of large wall thickness is assigned. The preform section assigned to the functional maximum subject to strong stretching in the blow mold. Each extreme the program curve set in individual areas is, the more precisely that belongs to these points Material that is subjected to the greatest stretching is in the right place in the blow mold be positioned. Deviations of more than ± 1% related to the height of the blow mold lead to considerable Loss of quality in the form of a hollow body distortion, lower compression values and lower strength properties of the finished hollow body. To ensure, that the preforms a predetermined target position relative to Take the blow mold and also that from the preforms manufactured hollow body have the specified target weight, regulation of the procedure is necessary, which at least depends on the speed of the screw extruder as well as the die gap of the extrusion head.

In a method known from EP-A-0 345 474 on the hollow body two different weight measurements carried out and according to the weight measurements. Screw speed and the die gap of the extrusion head controlled. Preferably the gross weight of the hollow body following removal from the blow mold as well as the net weight of the hollow body, according to the previous one Removal of the waste slugs, measured and this Measured values used for the control process. The procedure has proven itself in itself. Because the weight measurements due to the system, however, at the earliest after the fourth work cycle are present, namely after preform formation (Bar 1), widening in the blow mold (bar 2) and removal the waste slug (bar 3), the procedure reacts for disturbances in the extrusion, for example as a result changed melt temperatures, fluctuations in the delivery rate, Changes in material properties, and the like, with a considerable time delay. In the meantime preforms produced in the second to fourth work cycle can lead to defective hollow bodies. The Requires two weight measuring devices also a considerable investment in plant technology.

In a method known from EP-A-0 026 828, one is optical measuring device for detecting the lower edge of the extruded preforms provided in a given Distance from the die gap of the extrusion head is positioned and upon detection of the lower edge of the preform controls the movement of the blow mold. The time to which the preform reaches its target position is also measured and compared with a target time. In accordance with the setpoint deviation, the speed of the Screw extruder regulated, with the aim of keeping the time in which a preform is formed, as constant as possible to keep. Furthermore, the weights of the hollow body measured. Weight deviations occur from a target weight on, the nozzle gap of the Extrusion head. Because of the interaction between the Several control loops are required in the parameters in a specific order. Has priority Detection of the hollow body weight. Becomes a weight deviation determined, a correction of the Nozzle gap. There is a change in the length of the preform, which in turn is a change in hose length-dependent controlled cycle time of the machine. With The length change is made with the help of a subordinate control loop of the preform by correcting the Extruder speed compensated again and the cycle time of the Blow molding system adapted to a setpoint. A third subordinate Loop is activated when there are differences between the cycle time of the blow molding system and the cycle time the wall thickness programmer due to a too low selected step size for the extruder speed correction occur. The regulatory process works, in particular with regard to correcting the position of the preform, slowly. The regulatory process is complex.

In addition, it is known that the blow mold is independent of the Timing preform length. The sequence of movements the blow mold and its parts are controlled in a timely manner.

Practice has shown that individual blown products are one Blow molding system controlled according to the state of the art. can have considerable differences in compression value. If limited the wall thickness range to narrow areas, distinctive functional maxima are the differences extraordinary big. To achieve high compression values with a minimal operating weight, these are pronounced Functional maxima are required. The more extreme that Functional maxima are pronounced, the more important is the Compliance with the blow mold position. Will be at random individual hollow bodies are below the minimum compression values the whole production batch is determined unusable because of an insufficient compression value In practice, a single hollow body can be the cause be sure that, for example, a stack of filled Canisters tip over. In order to operate in the process to be on the safe side is helpful in practice often with the curve of the wall thickness program below Reduction of the functional maxima, while broadening the functional tips, is sanded, whereby increased material requirements or lower compression values in purchase be taken. It is unsatisfactory.

The invention has for its object a method for extrusion blow molding of hollow bodies at the beginning Specify the type described in the case of weight deviations the hollow body as well as deviations from the given. Distribution of materials in the hollow bodies, in particular in Hollow body sections in the blow mold a particularly subject to strong stretching, quickly and safely corrected can be.

zu jedem Vorformling die bis zum Erreichen einer vorgegebenen Schlauchlänge benötigte Extrusionszeit gemessen, deren Sollwertabweichung von einem Sollwert bestimmt und die Sollwertabweichung zusammen mit dem Istwert für die Drehzahl und/oder der Drehzahländerung des Schneckenextruders sowie dem Istwert für den Düsenspalt und/oder der Düsenspaltänderung unter Zuordnung zum Vorformling und daraus hergestelltem Hohlkörper in einem Rechner gespeichert wird,

die Gewichte der Hohlkörper gemessen und Gewichtsabweichungen von einem vorgegebenen Sollgewicht mit den zugeordneten, gespeicherten Sollwertabweichungen der Extrusionszeit paarweise als Regelabweichungen für die Festlegung erforderlicher Drehzahl- und Düsenspaltkorrekturen eingesetzt werden und

aus den gespeicherten Drehzahl- und Düsenspaltwerten sowie den ermittelten Drehzahl- und Düsenspaltkorrekturen die Betriebsparameter für die Extrusion in einem folgenden Arbeitstakt festgelegt werden.

The object of the invention and solution to this problem is a process for the extrusion blow molding of hollow bodies made of thermoplastic material, in which tubular preforms are extruded from the die gap of an extrusion head controlled according to a wall thickness program by means of a screw extruder and the extruded preforms are expanded in a blow mold into hollow bodies in the following working cycles. the hollow bodies are formed and adhering waste slugs are removed, wherein

For each preform, the extrusion time required to reach a specified hose length is measured, its setpoint deviation is determined from a setpoint and the setpoint deviation is assigned together with the actual value for the speed and / or the speed change of the screw extruder and the actual value for the die gap and / or the die gap change the preform and the hollow body produced therefrom is stored in a computer,

the weights of the hollow bodies are measured and weight deviations from a predetermined nominal weight with the assigned, stored nominal value deviations of the extrusion time are used in pairs as control deviations for determining the required speed and nozzle gap corrections and

the operating parameters for the extrusion are determined in a subsequent work cycle from the stored speed and nozzle gap values and the determined speed and nozzle gap corrections.

The paired use of extrusion time and Weight measurements on the same object in different production levels are detected, form a set of information that enables the Screw speed and the nozzle gap at the same time change that most of the time that follows extruded preforms the specified extrusion time and the hollow body produced from it the given one Have target weight. It is within the scope of the invention the weight of the net weight of the hollow body after Removing the waste slugs or the gross body weight, which immediately after demolding of the hollow body is measured from the blow mold, or also another for the blow product taken from the blow mold characteristic reference weight, which is also the Can represent the sum of partial weights. in the The scope of the invention is also the weight of the lower waste as a weight measurement. There the extrusion time in a previous time cycle the necessary regulations can be determined be carried out as soon as the still required Weight measurement is available. Are disturbances in this way so early by changing the screw speed and the Correctable nozzle gap.

Another improvement of the method according to the invention can be achieved in that a - for example at Start-up, batch change or optimization - new ones Setpoint deviation of the extrusion time, which a exceeds the given limit in the next work cycle regardless of the weight measurement not yet available by changing the screw speed and / or is eliminated by changing the nozzle gap. Will one Setpoint deviation found, the correction of Control variable that influences the situation, priority. The Correction is made so that the preform sections that subject to particularly strong stretching in the blow mold, take their target position. An alternative embodiment of the method according to the invention provides that a manipulated variable for the preform formation correcting the position of the preform is activated if the setpoint deviation of the measured extrusion time one exceeds the predetermined limit, being used for the measurement the extrusion time a measuring device is used, the along the extrusion path at a distance from the lower edge of the preform of the fully extruded preform is arranged. The manipulated variable for the position correction of the The amount of the preform is such that the assigned a functional maximum of the wall thickness range Preform cross section positioned in the blow mold there is where a maximum assigned to the maximum function value Stretching occurs during the blowing process.

The embodiments of the invention described above allow particularly quick regulation of the preform position. The rule intervention has only provisional Character. As soon as the one belonging to the extrusion time Weight measurement is available a few clock cycles later the necessary speed and nozzle gap corrections made that in the subsequent preforms not only the extrusion time but also its weight or that Weight of the hollow bodies made from the preforms correspond to the target values. Wrong regulations, which resulted from the provisional intervention can be corrected in one step. To the preforms have this final correction not only the right position for blow molding but also that right weight. The invention is based on the consideration from that for the quality of a hollow body in the first place Line compliance with minimum compression values is crucial are and the precise adherence to the target weights on the other hand, it is of somewhat less importance. For Achieving a high compression value with a low operating weight are wall thickness programs with pointed ones Functional maxima required. If the procedure with a operated such a wall thickness program, it is crucial to the fact that the assigned to the functional maxima Preform sections their target position to the blow mold take in. The disadvantage that some of the hollow bodies in the train manufacturing can have hollow body weights that may differ slightly from the target weight, however can easily be accepted.

In the practical implementation of the method according to the invention one will expediently change the Only start the screw speed and the nozzle gap if the setpoint deviations of the extrusion time in one given by internal limits and external ones Limit values fall within the tolerance range, whereby the internal limits with a narrow band range positive and negative deviations from the setpoint of the Limit extrusion time and take the outer limits maximum permissible positive and negative deviations from define the target value of the extrusion time.

Various options are available for the further configuration of the method according to the invention. In a first embodiment of the invention, the speed of the screw extruder is changed in accordance with the weight deviations from the predetermined target weight and, in addition, the nozzle gap is corrected in accordance with the weight deviations in conjunction with the assigned target value deviations of the extrusion time. On the basis of linear dependencies, the speed change Δ n _{S of} the screw speed n _S and the adjustment Δ s _{A of} the die gap s _A as a function of the extrusion time and weight measured values t _actual , G _actual and the assigned target values t _target , G _target according to the following algorithm Δ n s = n s • (G Should - G Is ) G should Δ s A = s A • (1- t Is t Should • n s n s + Δn s ) performed. The gross weight of the hollow body, including the adhering waste slugs, measured after the removal from the blow mold, is preferably used as the measured weight G _actual . However, it is also within the scope of the invention to use other weight measurements for the control method, in particular the net weight or another comparative weight which is characteristic of the blow product removed from the blow mold and which can also represent the sum of partial weights.

According to another embodiment of the invention, after Determination of the weight deviations from the specified Corrected target weight of the die gap of the extrusion head and is in accordance with the weight deviations in Connection with the assigned setpoint deviations of the Extrusion time an additional change in screw speed performed.

Based on linear dependencies, the adjustment of the nozzle gap Δs _A and the speed change Δn _{S of} the screw speed Δ n _S are dependent on the extrusion time and weight measurements t _actual , G _actual and the assigned target values t _target , G _target according to the following algorithm Δ s A = s A • (G Should - G Is ) G should Δ n S = n S • (1- t Should t Is • s A s A + Δs A performed. The net weight of the hollow body after removal of the waste slugs is preferably used here as the measured weight G _actual . However, the gross hollow body weights or other comparison weights, which can also represent the sum of partial weights, can also be used.

For the arrangement of the measuring device along the Exist extrusion path for measuring the extrusion time various possibilities. According to a first embodiment the extrusion time by a measuring device captures that near the bottom of the completely extruded preforms is positioned. The distance the measuring device from the nozzle gap is chosen such that even if the preform is not the desired length still reaches a measurement signal is given.

Another advantageous embodiment of the invention The method provides that the extrusion time by a Measuring device is detected, its distance from the nozzle gap is dimensioned so that this distance of the target length of a preform section from the preform lower edge or a reference mark up to one critical preform cross-section that a functional maxium assigned to the wall thickness program. In this embodiment of the method according to the invention the extrusion time measurement a function maximum of Wall thickness program directly assigned. Equals to Extrusion time measured value to the setpoint, so is exactly at this point the preform section extrudes the subject to particularly strong stretching in the blow mold. Since in this embodiment of the method according to the invention timing not at the end of the preform, that is, after the extrusion has been completed, surveillance should ensure that: the residual extrusion of the preform after the measurement is not is still disturbed. In a further embodiment, the Invention therefore that the program-compliant process of Residual extrusion is checked and an interference signal is emitted becomes when a. during the residual extrusion of the preform Disorder occurs, the one from the concerned Preform manufactured hollow body due to the interference signal is removed. The monitoring of residual extrusion is possible in different ways. For example, if the im extrusion time measured in the next working cycle - within the framework the permissible, operational deviations - those before corresponds to the measured extrusion time, so is the residual extrusion been carried out without interference. The supervision the residual extrusion can therefore be compared that measured in successive work cycles There are extrusion times. To monitor residual extrusion can also an additional time measuring device near the bottom edge of the fully extruded one Preforms are arranged, these additional Time measurement device triggers the interference signal when the time measurement value specified limit values reached.

If there is a shift in the critical cross section of the Preform is determined relative to the blow mold, although the weight and extrusion time readings do A fine correction is advisable if it corresponds to the target values.

For the purpose of fine correction, the relative movement changed between the blow mold and the preform and / or the sequence of the wall thickness program relative to Preform moved or with a short-term Provide a stopover at a specified profile point and / or the molding time is changed or a different one comparable time correction can be made. Further can be used for a fine correction in the wall thickness range the length of the program curve section corresponding to the section between the bottom of the preform and one with respect to the later stretching of the blow mold Preform critical cross section of the preform is assigned to be changed.

In a further embodiment, the invention teaches that Lower edge of the preforms by at least one light barrier is recorded, which is the timing of the Extrusion time stops. Preferably there are several light barriers provided in a horizontal plane, wherein by averaging that of the light barriers stopped time measurements for the setpoint comparison used extrusion time (actual time) is determined. In Another embodiment teaches the invention that the several horizontally arranged light barriers stopped measured time values compared and that an error signal associated with the preform is saved when the time difference between the stopped time measurement values outside of a predetermined one Tolerance range. The error signal can be discharged the molded from the preform in question Hollow body and / or as a controlled variable for changing a Operating parameters of the extrusion blow molding process used become. The time sequence of the error signal underlying measurement signals allow conclusions to be drawn about the Shape of the preform bottom edge and allow that Detect operational failures. A distracting Hose misalignment is as identifiable as one too large or too small amount of supporting air. According to preferred Execution of the method according to the invention is therefore in accordance with the chronological order of the error signal underlying measurement signals the amount of supporting air or a slanted hose run corrected. A great measurement accuracy and reliable evaluation of the measurement signals is particularly important then ensured when the light barriers in Groups are arranged horizontally at an angle of 90 ° are preferably aligned with each other and each include at least two parallel light barriers. The measurement signals are described in the above Way evaluated. In addition, by averaging of the time measurements stopped by the light barrier determines the setpoint comparison used extrusion time become.

It goes without saying that in place of light barriers correspondingly other measuring devices for Detection of the lower edge of the preform and / or one on the Preform attached marking can be used can whose measurement signals in the manner described be evaluated. The timing can finally synchronously or at different times from the start of the Working cycle or by a measurement signal one below the Nozzle gap arranged optical measuring device, the e.g. B. a mark on the preform or the Preform lower edge detected, started.

It is within the scope of the invention to have several measuring devices in the direction of preform extrusion for the purpose of position control, Provide length adjustment or length control. If the material distribution in the hollow body is unsatisfactory is, although the extrusion time measurements and the assigned weight measurements correspond to the target values, are fine adjustments to the operating settings of the blow molding system expedient that in the description of the figures are explained.

In a further development of the method according to the invention the invention teaches that an associated with the preform Signal is saved when the setpoint deviation of the Extrusion time outside of a predetermined operational Tolerance field lies, and that this signal to discharge defective hollow body is used. Conveniently the signal controls a sorting device, if the from a preform with the signal blown hollow body reaches the sorting device, the hollow body in question is discharged. According to the invention, the extrusion time of a preform not only to control and regulate the process, but also as a quality characteristic of that from the concerned Preform produced hollow body used.

When starting up an extrusion blow molding machine are first considerable setpoint deviations in the extrusion time expect. As soon as the blow molding system is fully regulated and has reached a steady operating point is the average setpoint deviation of the extrusion time in general smaller. The operational tolerance area, which as Criterion for discharging hollow bodies is used, can be adjusted. During the start-up phase or a material, batch or color change, it is advisable while accepting a somewhat increased material requirement the mean wall thickness of the preforms and thus the weight of the hollow body by a few percentage points to make sure that even with one wider operational tolerance and larger allowable Setpoint deviations of the extrusion time required strength properties of the hollow body achieved become. The invention teaches in this context in a further advantageous embodiment that the Setpoint deviations of the preforms frequency distributions created and saved and that the operational tolerance, which is the criterion for Ejection of hollow bodies is used depending changed by the applicable frequency distribution becomes. Depending on the breadth of the operational The target weight is preferably also the tolerance field changed, with a narrow tolerance field a smaller one Target weight and a wider tolerance field a larger one Target weight is to be assigned. An increase in the target weight by a few percentage points compared to a target weight, which with fully regulated blow molding system and steady state is aimed for is Appropriate starting procedure. The dependency between that Target weight, the operational tolerance area and the Frequency distribution of the setpoint deviation is through empirical functions tangible and expedient in saved on a computer. When sending the signal to Removing defective hollow bodies are the in Computer stored functions taken into account.

It is also within the scope of the invention to change the internal limit values t _Tol-1 , t _{Tol + 1 of} the operational tolerance field, which form the criterion for initiating control corrections, as a function of the prevailing frequency distribution. Stochastic fluctuations of the time measurement values within the narrow band range limited by the internal limit values t _Tol-1 , t _{Tol + 1} are system-dependent. A very narrow band range can be used for regulated, quiet-running systems that are operated in the steady state. The assignment of the width of the band area delimited by the limit values t _Tol-1 , t _{Tol + 1} to the frequency distribution enables the band area to be continuously adapted to the system or the operation of the system.

Finally, it is within the scope of the invention that a optical and / or acoustic interference signal is emitted and the hollow bodies are removed if the setpoint deviation the measured extrusion time outside of an operational tolerance range reached.

The inventive method described above is characterized by considerable advantages. Allows starting a blow molding machine with automatic Transition to the production phase, allows optimization the hollow body and is of course also suitable for Material batch change and for the stationary Production plant. It allows you to work with one Wall thickness program, which is a program curve with extreme inserted functional maxima. According to the invention Processed hollow bodies stand out due to high minimum compression values with minimum hollow body weight out. Disturbances can be corrected quickly and specifically are minimizing rejects. Furthermore, one Control and discharge of hollow bodies guaranteed which do not reach the required mechanical values. Finally, the method according to the invention enables Manufacture of canisters and other hollow bodies with complicated shape in continuous extrusion. The method is especially for the production of Suitable plastic fuel tanks where the Maintaining minimum wall thicknesses is the main problem.

Fig. 1

ein Anlagen- und Regelschema zum Extrusionsblasformen von. Hohlkörpern aus thermoplastischem Kunststoff,

Fig. 2

die Häufigkeitsverteilung für die Sollwertabweichung von der Extrusionszeit bei einer nach dem erfindungsgemäßen Verfahren betriebenen Blasformanlage im Vergleich zum Stand der Technik,

Fig. 3

eine der Vorformlingsunterkante zugeordnete Meßeinrichtung,

Fig. 4

den Schnitt A-A in Figur 3.

In the following the invention will be explained with reference to a drawing showing only one embodiment. They show a schematic representation

Fig. 1

a system and control scheme for extrusion blow molding of. Hollow bodies made of thermoplastic,

Fig. 2

the frequency distribution for the setpoint deviation from the extrusion time in a blow molding system operated according to the inventive method in comparison to the prior art,

Fig. 3

a measuring device assigned to the lower edge of the preform,

Fig. 4

the section AA in Figure 3.

1 shows that tubular preforms 1 are extruded from the die gap 4 of an extrusion head 5, which is controlled according to a wall thickness program 3, by means of a screw extruder 2. The extruded preforms 1 are then expanded into hollow bodies 7 in a blow mold 6. The hollow bodies 7 are formed and adhering waste slugs are removed. For each preform 1, the extrusion time t _actual required to reach a predetermined tube length _is measured, its setpoint deviation Δt is determined from a setpoint t _setpoint , and the setpoint deviation Δt together with the actual values for the speed n _{S of} the screw extruder 2 and the die gap s _A stored in a computer 8. The storage takes place with assignment to the preform 1 and the hollow body produced therefrom. In addition, the weights G _{Ist of} the hollow body 7 are measured, namely their gross weights G _B or their net weights G _N. In this case, weight determined deviations Δ G from a predetermined desired weight W _{is to} be with the associated stored setpoint deviations Δ t the extrusion time _t in pairs as control deviations for determining the required speed and die gap corrections Δ n _S, Δ s _A used. The operating parameters n _S , s _A for the extrusion are determined in the following working cycle from the stored speed and nozzle gap values and the determined speed and nozzle gap corrections Δ n _S , Δ s _A.

The speed change Δ n _{S of} the screw speed n _S and the adjustment Δ s _{A of} the nozzle gap s _A is dependent on the extrusion time and weight measured values t _actual and G _actual and the assigned target values t _target , G _target according to the following algorithm Δ s A = s A • (G Should - G Is ) G should Δ n S = n S • (1- t Should t Is • s A s A + Δs A performed. From the specified algorithm it can be seen that the nozzle gap Δ s _{A is} changed in accordance with the change in weight Δ G from the predetermined target weight G _Soll . In addition, the speed Δ n _{S of} the screw extruder is additionally corrected in accordance with the weight deviations Δ G, but in conjunction with the assigned set point deviations of the extrusion time Δ t. Speed and nozzle gap corrections Δ n _S , Δ s _A are carried out simultaneously. The net weight G _N is preferably used as the weight measurement.

The extrusion time t _actual is recorded by a measuring device 9, which is positioned in the area of the lower edge of the preforms 1 after their complete extrusion and has at least one light barrier which stops the time measurement of the extrusion time t _actual . The distance of the measuring device 9 from the nozzle gap 4 corresponds approximately to the desired length of the preforms 1, but is selected so that even if the actual preform length is somewhat shorter than the desired length, a measurement signal is still emitted. The measuring device 9 expediently comprises a plurality of light barriers. The extrusion time t _actual used for the setpoint comparison is determined by averaging the time measured values stopped by the light barriers. In this way, measurement errors due to irregularities on the lower edge of the preform can be largely eliminated.

In the preform extrusion direction are for the purpose of Position control, length adjustment or length control further measuring devices 10a, 10b are provided. Divided by two or more measuring devices 10b arranged one below the other, 10a, 9 and an extraction time comparison between the Start and the measuring equipment can have operating parameters to compensate for sag and / or shrinkage be determined.

If the setpoint deviation Δ t of the measured extrusion time outside a given operational tolerance range lies, is assigned to the preform 1 Signal stored, which is defective for ejection Hollow body is used. The signal controls to this end a sorting device 11 if the one with blown hollow bodies 7 'assigned to the signal reached the sorting device. The hollow body in question 7 'is then removed.

The table below shows, using examples, operating states which can occur in the method according to the invention and the resulting procedural measures. It is assumed that the target value t _target for the extrusion time is 12.0 seconds. If the measured extrusion time t _{Vfl is} in a range from 11.9 to 12.1 seconds, the preform takes up the predetermined position relative to the blow mold. Thickened wall sections of the preform, to which functional maxima of the wall thickness program are assigned, are positioned in blow molding areas in which the greatest stretching occurs. The hollow bodies made from these preforms have the required properties. If the measured extrusion time in a tolerance field between t _Tol2- (11.6 sec.) And t _{Tol2 +} (sec 12.4.), But outside of the target value t _set attributed band from 11.9 to 12.1 sec., Then the preform occupies an acceptable position in the blow mold and the hollow body made from it is usable. However, the screw speed n _S is regulated in accordance with the setpoint deviation Δ t and the extrusion parameters, in the exemplary embodiment the screw speed n _S , are corrected in this way. If the setpoint _deviation Δt lies outside the operational tolerance field t _Tol2- , t _{Tol2 +} specified by t _Tol2- and t _{Tol2 +} , a signal assigned to the preform is stored. The signal controls the sorting device 11 in the manner described for discharging defective hollow bodies when the hollow body in question has reached the sorting device. The screw speed is also regulated here in accordance with the setpoint deviation t. If the setpoint _deviation finally reaches the limit values t _Tol3- , t _{Tol3 +} which lie outside the operational tolerance _range , an optical and / or acoustic interference signal is emitted without a time delay and all hollow bodies are removed.

FIG. 2 shows the frequency distribution for the setpoint deviation .DELTA.t from the extrusion time _t.sub.act in a blow molding system operated according to the inventive method in comparison with the prior art. The bar graph 12 relates to the method according to the invention. The distribution curve B shows a frequency distribution which is measured in a blow molding system operated according to a known open-loop and closed-loop control method. It can be achieved by the method according to the invention that the deviations Δ t from the extrusion time lie in a comparatively very narrow band. This narrow frequency distribution when using the method according to the invention is based on the fact that disturbances which influence the position of the preforms in relation to the blow mold can be corrected quickly and in a targeted manner.

The measuring device 9 comprises a plurality of light barriers 14, which are arranged horizontally next to each other. At a comparative view of FIGS. 3 and 4 one that the light barriers 14 are arranged in groups, the horizontally at an angle of preferably 90 ° are aligned with each other and at least two each include parallel light barriers. In the embodiment are the groups in different horizontal levels arranged. Photoelectric sensors are intended within the scope of the invention Photo sensors as well as other contactless working Measuring devices for the position detection of the preform are suitable include. The one from the light barriers 14 stopped time measurements are compared, and it becomes an error signal associated with the preform saved when the time differences between the stopped time measurement values outside of a predetermined Tolerance range. 3 is, for example annoying hose misalignment shown. From the temporal Sequence of the basis of the error signal This signal can be recognized by measurement signals and can therefore be Operating personnel optically and / or acoustically displayed and / or corrected by intervening in the control process become. A disturbing hose misalignment reduces the Quality of the preform 1 made Hollow body 7. Consequently, the error signal also to discharge the preform from the concerned 1 shaped hollow body 7 used. In addition can also use the error signal as a controlled variable for change an operating parameter of the extrusion blow molding process be, e.g. B. to change the temperature of a or several heating or cooling segments of the extrusion head 5 or heating or cooling devices on the preform 1. Not only annoying hose misalignment but also a quantity of supporting air deviating from the setpoint is included With the help of several light barriers 14 Measuring device 9 can be detected and by intervention in the Control procedures by hand or, if necessary, automatically Getting corrected.

The wall thickness program 3 divides the preform volume in n equidistantly distributed wall thickness profile points. Preferably with n = 25 wall thickness profile points worked. The program curve of the wall thickness program 3 points to narrow areas, pronounced Function maxima KP1, KP2 at a distance y1, y2 (from which in Direction of extrusion viewed from the front end of the preform) on the preform areas assigned which are strongly stretched in the blow mold. These preform cross sections are therefore also called critical cross sections. The program curve of the wall thickness program 3 is composed of one in the embodiment uniform basic gap 13, a ramp 14 and one Profile curve 15. The ramp 14 increases from the beginning of the Preform extrusion at the die gap 4 of the extrusion head 5 continuously. The ramp 14 helps sagging of the preform 1 due to the length-dependent To compensate for weight.

The two measuring devices 10a, 10b are critical Points KP1, KP2 assigned and in one of the curve section y2-y1 corresponding to the program curve Distance arranged. The measuring devices 9, 10a are in a distance that the program curve section is assigned to y3. Reaches preform 1 at Extrusion process the measuring device 10a, then becomes the critical preform cross section KP2 produced in the nozzle gap 4.

• Relativbewegung zwischen der Blasform und dem Vorformling ändern;
• Ablauf des Wanddickenprogramms relativ zum Vorformling verschieben;
• Einformzeit ändern oder eine andere vergleichbare Zeitkorrektur vornehmen;
• Länge des Abschnitts y3 der Programmkurve korrigieren.

If the position control is not applied to the main critical point KP2 but to the preform length after almost complete extrusion of the preform 1 and a shift in the critical cross-section KP2 relative to the blow mold is found, although the weight and extrusion time measured values correspond to the target values, these are for the purpose of fine correction the following corrective measures are possible alone or in combination:

• Change relative motion between the blow mold and the preform;
• Shift the sequence of the wall thickness program relative to the preform;
• Change the molding time or make another comparable time correction;
• Correct the length of section y3 of the program curve.

The measures described above are also possible the residual extrusion of the program section y3 to control associated preform section.

a) einer linear steigenden Rampe,

b) einer exponentiell steigenden Rampe,

c) einem modifizierten Profilfaktor im Verhältnis zum Grundspalt,

d) einem Düsenspaltfaktor gleich Summe aus Grundspalt und Profilkurve multipliziert mit einem Profilfaktor,

e) einem reziproken Düsenspaltfaktor

zu beschreiben.Changes in the sagging are the next largest disturbance variable after fluctuations in the swelling and delivery rate under constant machine conditions. The sagging is, in a first approximation, a function of the current preform weight and the nozzle gap cross-section. The disturbances are different material properties and different melt temperatures. The more extreme the wall thickness profile in the preform has to be set, the more difficult it is to compensate for the sag, for example, by a linearly increasing ramp. If the material distribution in the hollow body is unsatisfactory, although the extrusion time measurement values and the assigned weight measurement values correspond to the target values, fine adjustments to the operating settings of the blow molding system are expedient, various measures being possible. As a first improvement, the invention recommends modifying the proportion of the profile factor and thus the profile curve in relation to the basic gap. The profile factor is multiplied by the function values of the program curve and stretches or compresses them. The profile factor can replace the use of a basic gap. Another improvement can be that the profile factor is used with a linear or exponentially increasing ramp. Finally, it is possible to approximate the sag by any combination

a) a linearly increasing ramp,

b) an exponentially increasing ramp,

c) a modified profile factor in relation to the basic gap,

d) a nozzle gap factor equal to the sum of the basic gap and the profile curve multiplied by a profile factor,

e) a reciprocal nozzle gap factor

to describe.

The sagging through another program field would be optimal similar to the wall thickness range, which is also synchronous to this expires, enter separately for each wall thickness point. Required changes are a fraction of the determined curve can be added automatically or subtractable.

If measuring devices 9, 10a and 10b, as indicated in FIG. 1, are used, one or more sections y1 / y2, y1 / y3 can be regulated when the wall thickness program is divided into sections. Furthermore, the preform length or the amount of supporting air and / or the supporting air pressure can be increased at a smaller distance y2-y1 and reduced accordingly at a larger distance. It is also possible to briefly interrupt the sequence of the wall thickness program at one or more profile points, preferably at the minimum between the function maxima KP1, KP2. Since the extrusion of the preform continues during the interruption of the program sequence, with the operating settings at the time of the program interruption, the preform is stretched in the middle area between the functional maxima. If the distance y2-y1 is too small, a correction is possible by reducing the profile factor and increasing the base gap or increasing the ramp. Reverse dependencies arise when the distance y2-y1 is too large.

Claims (29)

1. A process for extrusion blow moulding of hollow members from thermoplastic, wherein tubular preforms (1) are extruded by a screw extruder (2) from the die gap (4) of an extrusion head (5) controlled in accordance with a wall thickness program (3), and in subsequent work cycles the extruded preforms (1) are expanded in a blow mould (6) to form hollow members (7), which are taken out of the mould and any adhering flashes are removed, wherein

   the extrusion time (t_Ist) required by each preform (1) to reach a predetermined tube length is measured, the deviation (Δt) thereof from a set value (t_Soll) is determined, and the deviation (Δt) is stored in a computer (8) together with the actual speed (n_S) of the screw extruder (2) and/or the change (Δn_S) in the said speed and the actual nozzle gap (S_A) and/or the change (Δs_A) in the nozzle gap in association with the preform (1) and hollow members (7) made therefrom,

   the weight (G_Ist) of the hollow members (7) is measured and deviations (ΔG) from a preset weight (G_Soll) with the associated stored deviations (Δt) from the set extrusion time are used in pairs as control deviations for determining any corrections (Δn_S, Δs_A) required in the speed or the die gap and

   the operating parameters (n_S, s_A) for the extrusion are determined in a subsequent work cycle from the stored speed and die-gap values and the calculated speed and die-gap corrections (Δn_S, Δs_A).
2. A process according to claim 1, wherein a newly occurring deviation (Δt) from the set extrusion time exceeding a set limit (t_Tol-1, t_Tol+1) is eliminated in the next working cycle by changing the screw speed (Δn_S) and/or by changing the die gap (Δs_A) without regard to the not yet available weight measurement.
3. A process according to claim 1, wherein in time with the production of the preform, a value for correcting the position of the preform (1) is activated if the deviation (Δt) of the measured extrusion time from the set value exceeds a set limit (t_Tol-1, t_Tol+1) wherein the extrusion time is measured by a measuring device (10a, 10b) disposed along the extrusion path at a distance from the bottom edge of the completely extruded preform.
4. A method according to claim 3, wherein the value for correcting the position of the preform (1) is so chosen so that the preform cross-section associated with a function maximum (KP1, KP2) of the wall thickness program (3) is positioned in the blow mould (6) at the place where a maximum stretching associated with the function maximum occurs during the blowing process.
5. A process according to any one of claims 1 to 4, wherein changes in the screw speed (Δn_S) and in the die gap (Δs_A) are introduced only if the deviations (Δt) of the extrusion time from the set value are within a preset tolerance range defined by inner limits (t_Tol-1, t_Tol+1) and outer limits (t_Tol-2, t_Tol+2), wherein the inner limits (t_Tol-1, t_Tol+1) define a narrow band range with positive and negative deviations from the set value (t_Soll) of the extrusion time and wherein the outer limits (t_Tol-2, t_Tol+2) define maximum permissible positive and negative deviations from the set value (t_Soll) of the extrusion time.
6. A process according to any of claims 1 to 5, wherein the speed (n_S) of the screw extruder (2) is changed in dependence on the deviations (ΔG) from the preset weight (G_Soll) and wherein the die gap (s_A) is corrected in dependence on the weight deviations (ΔG) in conjunction with the associated deviations (Δt) from the set extrusion time.
7. A process according to claim 6, wherein the change (Δn_S) in the screw speed (n_S) and the adjustment (Δs_A) of the die gap (s_A) are made in dependence on the measured extrusion time and the measured weight (t_Ist, G_Ist) and the associated set values (t_Soll, G_Soll) in accordance with the following algorithm
8. A process according to claim 6 or 7, wherein Δ ns = ns • (GSoll - GIst)Gsoll SA = SA • (1- tIst tSoll • ns ns + Δns ) the weight measurement G_Ist is the gross weight (G_B) of the hollow member including the adhering flashes, measured on removal from the mould.
9. A process according to any of claims 1 to 5, wherein the die gap (s_A) of the extrusion head is corrected in dependence on the deviations (ΔG) from the set weight (G_Soll) and wherein the screw speed (n_s) is additionally changed in dependence on the weight deviations (ΔG) in conjunction with the associated deviations (Δt) of the extrusion time from the set value.
10. A process according to claim 9, wherein the die gap is adjusted (Δs_A) and the screw speed (n_S) is changed (Δn_S) in dependence on the measured extrusion time and weight (t_Ist, G_Ist) and the associated set values (t_Soll, G_Soll) in accordance with the following algorithm: Δ SA = SA • (GSoll - GIst)Gsoll ns = ns • (1- tSoll tIst • SA SA + ΔSA )
11. A process according to claim 9 or 10, wherein the measured weight (G_Ist) is the net weight (G_N) of the hollow member after removal of the flashes.
12. A process according to any of claims 1 to 11, wherein the extrusion time (t_Ist) is detected by a measuring device (9) positioned near the bottom edge of the completely extruded preform.
13. A process according to any of claims 1 to 11, wherein the extrusion time (t_Ist) is detected by a measuring device (10a, 10b) at a distance from the die gap (4) such that the said distance corresponds to the set length of a portion of the bottom edge of the preform or a reference marking up to a critical preform cross-section (KP1, KP2) associated with a function maximum of the wall thickness program (3).
14. A process according to any of claims 1 to 13, wherein for the purpose of precise correction, the relative motion between the blow mould (6) and the preform (1) is changed and/or the sequence of the wall thickness program (3) is displaced relative to the preform (1) or an intermediate stop is provided at a preset profile point and/or the moulding time is changed or another comparable time correction is made and/or the length of the curve portion (y3) of the wall thickness program (3) corresponding to the portion between the preform bottom edge and a critical cross-section (KP2) of the preform (1) relative to the subsequent stretching of the preform in the blow mould (6) is changed if a displacement of the critical cross-section (KP2) relative to the blow mould (6) is detected, even though the measured weight and extrusion time (G_Ist, t_Ist) correspond to the set values (G_Soll, t_Soll).
15. A process according to any of claims 1 to 14, wherein the bottom edge of the preform (1) is detected by at least one light barrier (14) which stops the measurement of the extrusion time.
16. A process according to any of claims 1 to 15, wherein the measuring device comprises a number of horizontal light barriers (14) disposed side by side and the time measurements stopped by the light barriers (14) are compared and wherein an error signal associated with the preform (1) is stored if the differences between the stopped time measurements are outside a preset tolerance range.
17. A process according to claim 16, wherein the error signal is used for ejecting the hollow member (7') moulded from the preform (1) in question and/or as a control variable for changing an operating parameter of the extrusion-blowing process.
18. A process according to claim 17, wherein the amount of back-up air or a skewed run of tubing is corrected in dependence on the sequence in time of the measured signals on which the error signal is based.
19. A process according to any of claims 15 to 19, wherein the light barriers (14) are disposed in groups aligned horizontally at an angle of preferably 90° to one another, each group comprising at least two parallel light barriers (14).
20. A process according to any of claims 15 to 19, wherein the time measurements stopped by the light barriers are averaged in order to determine the extrusion time (t_Ist) used for comparison with the set value.
21. A process according to any of claims 1 to 14, wherein during extrusion a marking is made on the outside of the preform (1) and wherein a device for detecting the marking is used for measuring the extrusion time used for comparison with the set value.
22. A process according to any of claims 1 to 21, wherein a number of measuring devices (10b, 10a, 9) are provided in the preform extrusion device for checking the position, setting the length or adjusting the length.
23. A process according to any of claims 1 to 22, wherein a number of measuring devices (10b, 10a, 9) are disposed one beneath the other in the extrusion direction and the extrusion times measured therewith are compared in order to obtain operating parameters for compensating sagging and/or shrinkage.
24. A process according to claim 23, wherein for the purpose of precise correction, the program curve (13, 14, 15) is corrected if the deviations (Δt) from the set value between the extrusion times measured by the measuring devices (10b, 10a, 9) differ from one another.
25. A process according to any of claims 1 to 24, wherein the time measurement is started in synchronism with or offset at each cycle from the beginning of the work cycle or by a signal from an optical measuring device disposed underneath the die gap (4) .
26. A process according to any of claims 1 to 25, wherein a signal associated with the preform (1) is stored if the deviation of the extrusion time from the set value is above a preset operating tolerance range (t_Tol2-, t_Tol2+) and wherein the signal is used for ejecting faulty hollow members (7').
27. A process according to claim 26, wherein the signal actuates a sorting device (11) when the hollow member (7') blown from a preform (1) designated by the signal reaches the sorting device (11), and the hollow member (7') in question is ejected as a result.
28. A process according to any of claims 1 to 27, wherein frequency distributions relating to the deviations (Δt) of the extrusion time (t_Ist) from the set value are compiled and stored and wherein the inner limits (t_Tol-1, t_Tol+1) and/or the outer limits (t_Tol-2, t_Tol+2) of the operating tolerance range are used in dependence on the current frequency distribution.
29. A process according to any of claims 26 to 28, wherein the set weight (G_Soll) is changed in dependence on the width of the operating tolerance range and wherein a smaller set weight is associated with a narrow tolerance range and a greater set weight is associated with a wider tolerance range.

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